1. Field of the Invention
The invention relates generally to an optical sensor arrangement for detecting the presence of a flame in a gas turbine engine. In particular, the invention is directed to a photodiode flame sensor having a variable sensitivity and simplified signal conditioning circuitry.
2. Related Art
A standard method for detecting the presence of a flame in a gas turbine engine has been to use a light activated or photosensitive tube, such as, for example, a Geiger-Mueller gas discharge tube. Such tube-based detectors typically include a phototube having a cathode that is phototransmissive, and an anode for collecting the electrons emitted by the cathode. The tubes are filled with a gas at low pressure that is ionized by any accelerated electrons. A large voltage potential, for example, 200-300 volts, is typically applied to, and maintained between, the cathode and anode, such that in the presence of a flame or light emitting a wavelength to which the tube is sensitive, photons of a given energy level will illuminate the cathode and cause electrons to be released and accelerated, thereby ionizing the gas.
Geiger-Mueller gas discharge tubes have a peak spectral response at approximately 200 nanometers. Emissions at this wavelength cause the gas in the tube to ionize as discussed above, causing a momentary pulse of current in the power supply. The frequency of these pulses is proportional to the ultraviolet intensity at low light levels. At higher levels, the output saturates at a frequency determined by the quenching time of the gas.
With the advent of low emission gas turbines, tubes have proven to be somewhat unreliable. The low emission turbines implement several methods to reduce emissions, including steam injection, water injection and pre-mixed fuels. All of these emission reducing methods tend to absorb ultraviolet radiation, thereby reducing the signal to the tube. Moreover, the Geiger-Mueller tube is a low frequency device that requires a long integration time, e.g., 125 milliseconds, before a decision as to flame status can be made.
Another system for flame detection, specifically for detecting the presence of afterburner flame in augmented gas turbine engines is disclosed in U.S. Pat. No. 4,510,794 to Couch. The Couch system relies on an ion/electrostatic probe that provides ionic flame detection and electrostatic engine wear monitoring by measuring the conductivity through the plasma of the afterburner flame.
Recently, modern electronic systems have replaced archaic tube-based hardware with semiconductor components, such as, for example, photodiodes. Photodiodes have been used in applications for measuring or detecting the presence of light throughout the visible spectrum and the ultraviolet spectrum. Their smaller size, greater stability, enhanced reliability and lower cost make them vastly superior to phototubes, such as, for example, Geiger-Mueller gas discharge tubes.
Generally, a photodiode is a p-n junction with an associated depletion region in which an electric field separates photogenerated electron-hole pairs, the movement of which generates a measurable current. When electromagnetic radiation of an appropriate magnitude strikes the semiconductor material of the photodiode, the electron-hole pairs are generated by photoconductive action. When these charge carriers are generated near a p-n junction, the electric field of the depletion region at the junction separates the electrons from the holes in the normal p-n junction fashion. This separation produces a short circuit current or open circuit voltage, typically referred to as the photovoltaic effect. Such photodiodes are of the type disclosed in U.S. Pat. No. 5,093,576 to Edmond et al.
U.S. Pat. Nos. 5,303,684 and 5,257,496 both to Brown et al. and commonly assigned to the assignee of the instant application, the disclosures of which are incorporated by reference in their entireties herein, disclose a combustion control system for controlling the level of NO.sub.x emissions produced in the combustion process to reduce such emissions, while maintaining a sufficiently high combustion flame temperature. This is achieved by monitoring the intensity of non-infrared spectral lines associated with the combustion flame and then dynamically adjusting the fuel/air ratio of the fuel mixture. These patents describe, in a general sense, the use of silicon carbide (SiC) photodiodes to measure light intensity in a system for generating a signal corresponding to the NO.sub.x emission concentration for adjusting the engine operation parameters.
U.S. Pat. No. 5,670,784 to Cusack et al. discloses a high temperature gas stream optical flame sensor for flame detection in gas turbine engines. The sensor includes a silicon carbide photodiode and silicon carbide based amplification hardware for generating a signal indicative of the presence of a flame. The photodiode and amplifier hardware are preferably disposed in a sensor housing. However, there is no disclosure in Cusack et al. of any means for adjusting the sensitivity of the photodiode detection circuit. Additionally, the processing circuitry associated with the disclosed sensor arrangement is unnecessarily complex.